Process for decarbonising a high-chromium steel melt

ABSTRACT

A process for decarburizing a steel melt for the production of high-chromium steels by blowing in oxygen in which the decarburization rate is continuously measured and the amount of oxygen to be injected is adjusted depending on the measured values. The following controlled quantities are calculated: 
     a) the duration of the Al--Si oxidation phase at the start of the decarburization process, 
     b) the duration of a principle decarburization phase immediately following the Al--Si oxidation phase until the transition point from the decarburization reaction to the metal oxidation is reached, and 
     c) the decarburization rate in the principal decarburization phase. The injected oxygen quantity is increased at an accelerated rate immediately following the Al--Si oxidation phase to the oxygen quantity of the principal decarburization phase until the decarburization rate calculated in c) is reached. The decarburization rate is maintained substantially constant for the duration of the principal decarburization phase by the injected quantity of oxygen. The injected oxygen quantity is continuously reduced immediately following the principal decarburization phase so that the decarburization rate decreases continuously in time at a predetermined time constant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to a process for decarburizing a steel meltfor the production of high-chromium steels by blowing in oxygen in whichthe decarburization rate is continuously measured and the amount ofoxygen to be blown in is adjusted depending on the measured values. Thedecarburization rate is determined from the CO content and CO2 contentin the exhaust gas and from the flow of exhaust gas.

2. Discussion of the Prior Art

DE 33 11 232 C2 discloses a process for decarburizing steel melts inwhich the process quantities by which the decarburization process is tobe controlled are calculated on the basis of a theoretical modeldescribing the course of decarburization in the steel melt. For thispurpose, oxygen and a diluting gas are blown into the melt and theinjected quantities are controlled corresponding to the course ofdecarburization by adjustable gas flow control means. The controlling ofthe injected quantities is carried out so that the extent ofdecarburization and the carbon content of the melt during the meltingprocess is calculated with reference to the model and is compared withpredetermined values. When the calculated value agrees with thepredetermined value, the proportion of dilution gas and the gas quantityinjected into the melt are changed in a predetermined manner.Accordingly, in this process, the characteristic quantities in themodel, i.e., those inputted in the computing program, are compared withactual measured quantities and, by comparing the predetermined referencevalues and the calculated actual values, the control of thedecarburization process is carried out so that the actual course of theprocess corresponds as far as possible to the course of the processsimulated in the computer. The decarburization process can be controlledexactly by this computer-controlled decarburization process.

While this process is suitable for the decarburization of steel melts,this process based on the employed model is not suitable to determineexactly the time at which the point of transition from the decarburizingreaction to the metal oxidation is reached.

This results in increased chromium loss and accordingly additionallyrequired quantities of reducing materials, for example, ferrosilicon andlime, as basic neutralization of the silicon content in the slag, andfinally in a reduced life of the ladle or converter.

SUMMARY OF THE INVENTION

It is the object of the present invention to control, in an exactmanner, the decarburization of a steel melt for the production ofhigh-chromium steels by blowing oxygen into the melt such that, inparticular, unwanted chromium oxidation is avoided and a strongdecarburization of the melt and a minimum metal slagging are stillachieved.

Pursuant to this object, and others which will become apparenthereafter, one aspect of the present invention resides in a process fordecarburizing a steel melt for producing high-chromium steel. Theprocess includes the steps of injecting oxygen into the melt,continuously measuring a rate of decarburization and continuouslyadjusting the amount of oxygen injected depending upon the measuredvalues.

According to the invention, the following controlled variables arecalculated by means of a computer on the basis of measured orpredetermined values: the duration of the Al--Si oxidation phase at thestart of the decarburization process, the duration of a principledecarburization phase immediately following the Al--Si oxidation phaseuntil the transition point from the decarburization reaction to themetal oxidation is reached, and the decarburization rate in theprincipal decarburization phase, wherein the decarburization rate isdetermined in turn from the CO and CO2 content in the exhaust gas andthe exhaust gas flow.

The process is conducted so that the injected oxygen quantity isincreased at an accelerated rate immediately following the Al--Sioxidation phase to the oxygen quantity of the principal decarburizationphase until the calculated decarburization rate occurs. Subsequently,the decarburization rate is maintained substantially constant for theduration of the principal decarburization phase by changing the injectedquantity of oxygen. In the post-critical phase immediately following theprincipal decarburization phase, the injected oxygen quantity iscontinuously reduced in such a way that the decarburization ratedecreases continuously in time at a predetermined time constant.

In this way, a maximum decarburization and minimum metal slagging,especially a minimum unwanted chromium oxidation, under the givenconditions is achieved. The process according to the invention for theproduction of high-chromium steels makes use of the insight that thereis a critical decarburization state in the course of the process, thatis, a transition point from the decarburization reaction to the metaloxidation, which can be calculated with sufficient precision using aspecial model, and that conducting the process in an optimum manner isdependent on the timely detection of this state which, when exceeded,promotes metal oxidation, especially chromium oxidation, in the melt atthe detriment of the decarburization reaction.

Only by determining the critical decarburization state is it possible topredict the process sequence over time as it relates to managing theprocess. When the input data of the preliminary metal are known,especially the chemical composition, the temperature the and weight, andthe presetting of desired end data in the same form as the input data ofthe melt, the important variables for conducting the process withrespect to regulation technique can be calculated beforehand withreference to the model.

A specific arrangement of the model for determining the criticaldecarburization state which makes it possible to determine the durationof the Al--Si oxidation phase ΔtAl--Si, the duration of the principaldecarburization phase Δtkr, and the decarburization rate in theprincipal decarburization phase is described by equations (1) to (5).This model assumes that during the principal decarburization phase, avirtually constant decarburization rate exists which, after thetransition point from the decarburization reaction to metal oxidation isreached, passes into the immediately following post-critical phase. Inthis connection, the oxygen supply multiplied by the efficiency of theoxygen lance in the principal decarburization phase is constant.

A very small Cr loss is achieved in that the oxygen supply is reducedcontinuously over time as the decarburization rate decreases at the timeconstant τkr calculated by means of equations (1) to (5).

The control can be realized in a very simple manner by blowing in oxygenwith adjustable gas flow control means.

In conducting the decarburization process, it is proposed that thequantity of the injected oxygen be adjusted to a predetermined flowquantity for the duration of the Al--Si oxidation phase, so that thefoaming of the slag does not exceed a determined intensity.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention is explained more fully with reference tothe accompanying drawing.

FIG. 1 shows the decarburization kinetics of the model serving as basis;and

FIG. 2 shows the oxygen balance of the decarburization kineticsaccording to FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically the decarburization kinetics of the basemodel. The decarburization rate is plotted on the y axis and the carboncontent of the melt is plotted on the x axis. As is shown by FIG. 1, theprincipal decarburization phase is characterized by a constantdecarburization rate which passes continuously into the post-criticalphase after the critical transition point from the decarburizationreaction to metal oxidation is reached. From this view point, thecritical transition point is associated both with the principaldecarburization phase and with the post-critical phase. Accordingly, thedifferent kinetics of the decarburization reaction applicable to bothphases are identical, i.e.:

    ΔCkr/Δtkr=Ckr/τkr                          (1),

where

ΔCkr is the carbon loss until the critical point in %,

Δtkr is the duration of the principal decarburization phase,

Ckr is the critical carbon content in %,

τkr is the operation reaction time constant in minutes.

The actual decarburization takes place during the principaldecarburization phase, i.e., after the Al--Si loss until reaching thecritical transition point. As is well known, metal oxidation,principally oxidation of chromium, manganese, and iron, takes placeparallel to the carbon oxidation. This results in the following equationfor the oxygen balance:

    ΔO2,C+ΔO2,Me=ηHQO2,H Δtkr            (2),

where

ΔO2,C is the oxygen requirement for carbon loss until the critical pointin Nm3/min,

ΔO2,Me is the oxygen requirement during metal loss until the criticalpoint in Nm3/min,

ηH is the efficiency of the oxygen lance in the principaldecarburization phase,

QO2,H is the quantity of the injected oxygen in the principaldecarburization phase in Nm3/min

The appearance of the energy balance of the melt is such that theinstantaneous energy content of the melt is composed of the initialenergy content of the pre-metal and of the stored energy which is equalto the difference between the energy supply and the energy loss.Further, it is assumed that the reference temperature of the meltreached first at the critical point only increases slightly duringfurther processing in the post-critical phase. The proposed processcontrol in which only a slight chromium slagging occurs during thepost-critical phase is based on the above assumption. The release ofenergy during the carbon and chromium loss is compensated for the mostpart by the occurring energy loss. The energy balance is accordingly asfollows: ##EQU1## where GA is the weight of the melt in kg

ΔSi is the Si loss, where const1=25 to 40 K/0.1% Si loss

ΔAl is the Al loss, where const2=25 to 45 K/0.1% Al loss

ΔCkr is the C loss, where const3=5 to 20 K/0.1% C loss and A is theproportion (const4=20 to 40) of the CO subsequent combustion

ΔCrkr is the Cr loss, where const5=5 to 20 K/0.1% Cr loss

ΔFekr is the Fe loss, where const6=1 to 10 K/0.1% Fe loss

ΔMnkr is the Mn loss, where const7=5 to 20 K/0.1% Mn loss

CTP is the specific heat capacity of the melt in KWh/K/t

λ is the proportion of CO subsequent combustion in the vessel

CGP is the specific heat capacity of the waste gas in KWh/Nm3/K

QAr,Al--Si, QAr,H is the Ar inert gas flow in the Al--Si and principaldecarburization phase in Nm3/min

CWP is the specific heat capacity of the cooling water in KWh/l/K

ΔTw is the temperature difference between inlet and outlet in K

QW is the mean cooling water flow in l/min

CSP is the radiation output of the wall in KW

Gi is the feed "i" in kg

Ci is the enthalpy of the alloy "i" in KWh/t

T0 is the temperature of the premetal in ° C.

The right-hand side of the energy balance equation (3) has several termsprovided with a positive mathematical sign which account for the thermalenergy released through the metal loss (metal oxidation). The intensityof the metal loss is characterized, for the individual metals, by theconstants const. 1 to const. 7. This relates to typical parameters forthe melting furnace and the melt. The terms of equation (3) with anegative sign comprise the energy loss through the off-gas discharge,through the water cooling, through the heat radiation and the energyrequirement for melting in the alloys and slags.

The relationship between the temperatures relevant for the processfollows from equation (4):

    Tsoll=Tskr-T0                                              (4),

where

TSkr is the reference temperature of the melt at the critical point in °C, and

ΔTsoll is the reference temperature increase in the melt at the criticalpoint in ° C.

T0 is the temperature of the melt at the start of the treatment in ° C.

The essential quantity given by the solution to the equation system (1),(2) and (3) is the critical carbon loss ΔCkr. With this quantity, thecritical carbon content ΔCkr which is the carbon content at thetransition point of the melt according to FIG. 1 is given by thefollowing equation:

    Ckr=CA-ΔCkr                                          (6),

wherein CA is the initial carbon content of the melt.

The decarburization rate can be calculated by taking into account thefollowing equation according to FIG. 1:

    (-dC/dt)=ΔCkr/Δtkr=Ckr/τkr                 (5).

In addition to the critical carbon content Ckr, the solution to theequation system (1)-(4) gives the process times tkr and tAl--Si whichare very important with respect to regulation technique. The fourthunknown determined by the equation system is the quantity (T0+ΔTsoll/2).Using this value in equation (4) gives Tskr--the reference temperatureof the melt at the critical point.

The model for determining the critical decarburization state is clearlydescribed by equations (1) to (5) and makes it possible to determine thecontrol quantities relevant for the decarburization process: theduration of the Al--Si oxidation phase ΔtAl--Si, the duration of theprincipal decarburization phase Δtkr, and the decarburization rate inthe principal decarburization phase.

The decarburization process is carried out in such a way that therelevant control variables are calculated at the start ofdecarburization by means of equations (1) to (5). The further processsequence is shown schematically in FIG. 2. In the Al--Si oxidationphase, a predetermined oxygen flow and a predetermined inert gas flow(for example, argon) are adjusted and conducted through the melt. Thepredetermined values are in a range in which the foaming of the metalslag does not exceed the permissable values. Immediately following theAl--Si oxidation phase, the inert gas supply is turned off and thesupplied oxygen quantity is increased at an accelerated rate until thedecarburization rate which is calculated for the principaldecarburization phase and which is determined from the CO and CO2content in the exhaust gas and from the exhaust gas flow occurs. Thisdecarburization rate is maintained substantially constant through theregulation of the oxygen supply during the principal decarburizationphase. When the critical transition point tkr is reached, the suppliedoxygen amount is reduced in proportion with respect to time at timeconstant tkr.

The special nature of the invention consists in that the metal bathconcentrations of the chemical elements, the metal bath temperature atthe critical point and the time of its occurrence are determined.Further, the chemical-thermodynamic ratios of the chemical reactionstaking place in the metal bath at the critical transition point arecalculated. With respect to the maximum instantaneous decarburizationand the minimum metal slagging, these reaction courses are optimum. Theoptimum reaction course is contained in the post-criticaldecarburization phase in that the process quantities calculated for thecritical transition point on the basis of the model are utilized forcontrolling the post-critical phase, so that the unwanted chromiumoxidation, oxygen consumption and consumption of reducing materials,especially silicon, can be substantially minimized. The oxygen flowquantity is controlled via the decarburization rate as in the principaldecarburization phase.

Moreover, the determination of the critical state with reference to themodel makes it possible to define the optimum input data of the melt.The possibilities for applying the process extend in principle to allprocesses which take place accompanied by reduced effect of carbonrelative to chromium oxidation. Such processes include the vacuumoxidizing process (VOD) and the AOD (Argon Oxygen Decarburization)converter process with all technical modifications.

What is claimed is:
 1. A process for decarburizing a steel meltcontaining Mn, Al and Si for producing high-chromium steels, comprisingthe steps of:injecting oxygen into the melt; continuously measuring arate of decarburization, including calculating as control quantities:a)a duration of an Al--Si oxidation phase at a start of decarburization,b) a duration of a principal decarburization phase immediately followingthe Al--Si oxidation phase until a transition point from adecarburization reaction to metal oxidation reached, the Al--Si phaseand the principal decarburization phase both having a flow of Ar inertgas, and c) a decarburization rate in the principal decarburizationphase; and adjusting an amount of oxygen injected depending onquantities measured and calculated during the measuring step, includingincreasing a quantity of injected oxygen at an accelerated rateimmediately following the Al--Si oxidation phase to an oxygen quantityof the principal decarburization phase until the decarburization ratecalculated in c) is reached, maintaining the decarburization ratesubstantially constant for the duration of the principal decarburizationphase by way of the injected quantity of oxygen, and continuouslyreducing the injected oxygen quantity immediately following theprincipal decarburization phase so that the decarburization ratedecreases continuously in time at a time constant.
 2. A processaccording to claim 1, wherein the calculating step includes calculatingthe duration of the Al--Si oxidation phase ΔtAl--Si, the duration of theprincipal decarburization phase Δtkr, and the decarburization rate inthe principal decarburization phase based on a model described by thefollowing equations (1) to (5):

    ΔCkr/Δtkr=Ckr/τkr                          (1),

where ΔCkr is carbon loss until the critical point in %, Δtkr is theduration of the principal decarburization phase, Ckr is critical carboncontent in %, τkr is the operation reaction time constant in minutes,

    ΔO2,C+ΔO2,Me=ηHQO2,H Δtkr            (2),

where ΔO2,C is oxygen requirement for carbon loss until the criticalpoint in Nm3/min, ΔO2,Me is the oxygen requirement during metal lossuntil the critical point in Nm3/min, ηH is efficiency of an oxygen lancefor injecting the oxygen in the principal decarburization phase, QO2,His the quantity of the injected oxygen in the principal decarburizationphase in Nm3/min, ##EQU2## wherein GA is weight of the melt in kg ΔSi isSi loss, where const1=25 to 40 K/0.1% Si loss ΔAl is Al loss, whereconst2=25 to 45 K/0.1% Al loss ΔCkr is C loss, where const3=5 to 20K/0.1% C loss and λ is a proportion (const4=20 to 40) of CO subsequentcombustion ΔCrkr is Cr loss, where const5=5 to 20 K/0.1% Cr loss ΔFekris Fe loss, where const6=1 to 10 K/0.1% Fe loss ΔMnkr is Mn loss, whereconst7=5 to 20 K/0.1% Mn loss CTP is specific heat capacity of the meltin KWh/K/t λ is a proportion of CO subsequent combustion in a meltvessel CGP is specific heat capacity of exhaust-gas in KWh/Nm3/KQAr,Al--Si, QAr,H is Ar inert gas flow in the Al--Si phase and principaldecarburization phase in Nm3/min CWP is specific heat capacity of thecooling water in KWh/l/K ΔTw is a temperature difference between inletand outlet in K QW is a mean cooling water flow in l/min CSP isradiation output of a vessel wall in KW Gi is feed "i" in kg Ci isenthalpy of the melt "i" in KWh/t T0 is temperature of the melt at astart of the treatment in ° C.where

    Tsoll=Tskr-T0                                              (4),

where TSkr is a reference temperature of the melt at the critical pointin ° C. ΔTsoll is the reference temperature increase in the melt at thecritical point in ° C.,where

    (-dC/dt)=ΔCkr/Δtkr=Ckr/τkr                 (5).


3. A process according to claim 2, including continuously reducing thedecarburization rate after reaching the critical point in time at timeconstant τkr.